Hydrogenated nitrile rubber, also referred to as “HNBR” for short, is produced by hydrogenation of nitrile rubber, also referred to as “NBR” for short.
For the purposes of the present invention, the term nitrile rubber refers to rubbers which are copolymers of at least one unsaturated nitrile and at least one conjugated diene and possibly further comonomers.
Hydrogenated nitrile rubber is a specialty rubber which has very good heat resistance, outstanding resistance to ozone and chemicals and excellent oil resistance.
The abovementioned physical and chemical properties of HNBR are combined with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found widespread use in a wide variety of applications. For example, HNBR is used for seals, hoses, belts and damping elements in the automobile sector, and also for stators, borehole extraction seals and valve seals in the field of oil and for numerous parts in the electrical industry, mechanical engineering and shipbuilding.
HNBR grades which are commercially available on the market usually have a Mooney viscosity (ML 1+4@100° C.) in the range from 55 to 105, which corresponds to a weight average molecular weight Mw, (determination method: gel permeation chromatography (GPC) using polystyrene standards) in the range from about 200 000 to 500 000. The polydispersity indices PDI (PDI=Mw/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight) measured here, which give information about the width of the molecular weight distribution, are frequently 3 or above. The residual double bond content is usually in the range from 1 to 18% (determined by IR spectroscopy).
The relatively high Mooney viscosity greatly restricts the processibility of HNBR. For many applications, an HNBR grade which has a lower molecular weight and thus a lower Mooney viscosity would be desirable. This would significantly improve the processibility.
Numerous attempts have been made in the past to decrease the chain length of HNBR by degradation. This degradation has been carried out, for example, via a mechanical route by means of mastication, e.g. on a roll mill. Chemical degradation by reaction with strong acids is also possible in principle. However, this chemical degradation has the disadvantage that functional groups such as carboxylic acid and ester groups are incorporated into the molecule and, in addition, the microstructure of the polymer is changed substantially. All these changes result in disadvantages in use.
The production of HNBR having a low molar mass, corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight of Mn<200 000 g/mol, is not possible by means of established production methods since, firstly, a step increase in the Mooney viscosity occurs on hydrogenation of NBR and, secondly, the molar mass of the NBR feedstock to be used for the hydrogenation cannot be reduced at will since processing in the available industrial plants is otherwise no longer possible because of excessive stickiness. The lowest Mooney viscosity of an NBR feedstock which can be processed without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the hydrogenated nitrile rubber obtained using such an NBR feedstock is in the order of 55 Mooney units (ML1+4 at 100° C.).
In the known prior art, this problem is solved by reducing the molecular weight of the nitrile rubber prior to the hydrogenation by degradation to Mooney values (ML 1+4 at 100° C.) of less than 30 Mooney units or a number average molecular weight of Mn<70 000 g/mol. The decrease in the molecular weight is achieved here by metathesis, with low molecular weight 1-olefins usually being added in this metathesis reaction. The metathesis reaction is advantageously carried out in the same solvent as the hydrogenation reaction (in situ) so that the degraded NBR feedstock does not have to be isolated from the solvent after the degradation reaction is complete, before it is subjected to the subsequent hydrogenation. The metathetic degradation reaction is catalysed using metathesis catalysts which are tolerant towards polar groups, in particular towards nitrile groups.
The production of HNBR having low Mooney values is described, for example, in WO-A-02/100941 and WO-A-02/100905. WO-A-02/100905 describes a process which comprises the degradation of nitrile rubber starting polymers by olefin metathesis and subsequent hydrogenation. Here, a nitrile rubber is reacted in the presence of a coolefin and a specific catalyst based on an osmium, ruthenium, molybdenum or tungsten complex in a first step and hydrogenated in a second step. According to WO-A-02/100941, it is possible to obtain hydrogenated nitrile rubbers having a weight average molecular weight (Mw) in the range from 30 000 to 250 000, a Mooney viscosity (ML 1+4@100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5 in this way.
Metathesis catalysts are known from, inter alia, WO-A-96/04289 and WO-A-97/06185. They have the following basic structure:
where M is osmium or ruthenium, R and R1 are each organic radicals having a wide structural variability, X and X1 are each anionic ligands and L and L1 are each uncharged electron donors.
Such catalysts are suitable for ring-closing metatheses (RCMs), cross metatheses (CMs) and ring-opening metatheses (ROMPs). However, the catalysts mentioned are not necessarily suitable for carrying out the degradation of nitrile rubber.
The metathesis of nitrile rubber can be carried out successfully using some catalysts from the group of “Grubbs (I) catalysts”. A suitable catalyst is, for example, a ruthenium catalyst having a particular pattern of substituents, e.g. the catalyst bis(tricyclohexylphosphine) benzylideneruthenium dichloride shown below.

After the hydrogenation, the rubbers have a very much lower molecular weight and a narrow molecular weight distribution (WO-A-02/100941, WO-A-03/002613).
US 2004/0110888 A1 discloses vulcanizates based on these low molecular weight HNBRs.
US 2004/0127647 A1 describes blends based on low molecular weight HNBRs having a bimodal or multimodal molecular weight distribution and also vulcanizates of these rubbers.
Furthermore, WO-A-00/71554 discloses a group of catalysts which are referred to in the art as “Grubbs (II) catalysts”. If such a “Grubbs (II) catalyst”, e.g. 1,3-bis(2,4,6-trimethyl-phenyl)-2-imidazolidenylidene)(tricyclohexylphosphine)ruthenium(phenylmethylene) dichloride, is used for the NBR metathesis, this succeeds even without the use of a coolefin. After the subsequent hydrogenation, which is carried out in situ, the hydrogenated nitrile rubber has a lower molecular weight and a narrower molecular weight distribution (PDI) than when catalysts of the Grubbs (I) type are used (US-A-2004/0132891).

The use of the nitrile rubbers which have been degraded using a Grubbs (II) catalyst for adhesive compositions is described, for example, in US 2004/0132906-A1.
The abovementioned processes for the degradation of nitrile rubber by metathesis and the catalysts used for this purpose have the disadvantage that solutions of these catalysts are unstable, especially in the presence of atmospheric oxygen, and always have to be made up fresh.
In addition, catalysts which will hereinafter be referred to as Piers (I) catalysts and have the following structure are also known in the art (cf., for example, Angew. Chem. Int. Ed. 2004, 43, 6161-6165):
where R is, for example, an isopropyl or cyclohexyl radical and A is an anion.
However, our own studies have shown that such Piers (I) catalysts do not lead to degradation of the nitrile rubber in NBR metathesis but instead lead exclusively to gelling of the NBR. This makes the nitrile rubber unusable.
Furthermore, WO-A-2005/121158 describes catalysts for metathesis reactions developed from the Piers (I) catalyst.